Exploring the Roles of
hlb-1, homologue of liprin protein SYD-2 Dayong Wang and Mei Zhen Samuel Lunenfeld Research Institute, Toronto, ON, Canada M5G 1X5 At synaptic junctions, the active zone, an electron-dense membrane structure, functions in vesicle docking and neurotransmitter release. Signaling mechanism that control the formation of the active zone and how active zone components regulate the neurotransmitter release, are not fully understood. SYD-2, a liprin family protein, localizes at the active zones, and restricts the area of active zones in C. elegans (Zhen and Jin, Nature 401:371-375, 1999). SYD-2 homologue in Drosophila may recruit receptor type tyrosine phosphatases to define areas for active zone development (Kaufmann et al., Neuron 34:27-38, 2002). It was also shown in vertebrates that a liprin protein directly interacts with RIM, another active zone component required for vesicle fusion (Schoch et al., Nature 415: 321-326, 2002). Despite its interaction with molecules essential for synaptic development and synaptic functions, the behavior phenotypes of
syd-2 null animals are very mild. For example, active zones only show a 2-fold increase in size, and neurotransmission, as indicated by aldicarb tests, is only moderately defective. It is therefore possible other liprin homologue in C. elegans plays redundant roles.
hlb-1(homologue of liprin-beta) encodes the only other liprin protein in C. elegans. HLB-1 protein has three coiled-coil domains, a putative trans-membrane domain and a SAM domain. Its SAM domain at the carboxy terminus is similar to that of SYD-2 protein, but its coiled-coil domain at the amino terminus is more divergent from that of SYD-2. We have generated a mutant in
hlb-1 gene with deletion of the carboxy terminal region. The
hlb-1 mutant animals exhibit a mild decrease in the thrashing frequency, moderate aldicarb resistance, and transient egg laying defects, suggesting that
hlb-1 may be required for neuronal function. In the nervous system, we did not find axonal abnormalities, whereas there might be a more diffuse SNB-1::GFP vesicle marker distribution. Furthermore, the
syd-2hlb-1 double mutant has more severe vesicle marker diffusion and acute aldicarb resistance than the
syd-2 null mutant, or the
hlb-1 mutants. We are analyzing the subcellular localization of HLB-1 protein, and if SYD-2 and HLB-1 functions together in presynaptic termini to regulate the development and function of active zones. (We acknowledge Dr. Joe Culotti and Sarah Bajurny for the deletion library and initial screen for
hlb-1 mutants)